Just mustard on my hot dog. I haven't eaten one in ages though. I also like the buns we used to get in MA. They had crust along the top and were bread on the sides. You put butter on the sides of the bun, grilled it and then put your hotdog in. YUMMY!
No ketchup for me. I worked with someone once who liked hotdogs with peanut butter on them. UCK!